Caution, Ordering The Right Size: Measure your existing filter because the size written on the frame may, or may not be the true size. Depending on the manufacturer, the size on your filter frame could be "Nominal" (by name only) or "Actual" (true size). For example, a 20x24 "Nominal" filter will measure 19 1/2 x 23 1/2. On the other hand, a 20 x 24 "Actual" filter will truly measure 20 x 24. If you need a "Nominal Size", but order an "Actual Size", the filter you get will not fit properly, and vice-versa.